1. Field of the Invention
The present invention relates to methods and apparatus for in-situ monitoring of a crystallization state to be carried out to monitor the crystallization state of a thin film in a process of anneal processing using an energy line (e.g., laser beam) as well as an annealing method and device using the method of in-situ monitoring of a crystallization state. The method and apparatus for in-situ monitoring of a crystallization state are used, for example, for monitoring the crystallization state when annealing (e.g., laser annealing process) an amorphous semiconductor thin film in a process for producing a thin film transistor for a switching element or device of a liquid crystal display or an organic electroluminescence (hereinafter to be called “EL”) display.
2. Description of the Prior Art
Conventionally, there has been a method of detecting the intensity of a reflected light from one locally irradiated place which is irradiated by an annealing area with a monitor light, as a method of monitoring crystallization of an amorphous silicon thin film formed on a glass substrate, for example, when crystallizing the film by laser annealing (See, e.g., Patent Document 1, Patent Document 2 and non-patent document 1).
Patent Document 1: JP Patent Appln. Public Disclosure No. 2001-257176
Patent Document 2: JP Patent Appln. Public Disclosure No. 11-148883
Non-Patent Document 1: “Excimer Laser-Induced Temperature Field in Melting and Resolidification of Silicon Thin Films” by M. Hatano, S. Moon, M. Lee, K. Suzuki and C. P. Grigoropoulos, in Journal of Applied Physics, Vol. 87, pp. 36-43, published in 2000
According to the foregoing non-patent document 1, a continuous wave laser beam (hereinafter to be called “CW laser”), that is, a helium neon (He—Ne) laser beam having a wavelength of about 633 nm, concretely, is used as a monitor light, the laser beam is applied to the thin film, a reflected light from the thin film is detected by a silicon PN junction photo diode type photo detector having a response time of 1 nanosecond (hereinafter to be called “ns”), and a temporal change of a detected signal waveform is measured by a sampling oscilloscope which samples a frequency signal of 1 GHz.
In this document, a pulsed laser beam, that is, concretely, a krypton fluorine (KrF) excimer laser beam in an ultraviolet area having a pulse width of about 25 ns (a value in full width at half maximum, hereinafter to be called “FWHM”) and a wavelength of about 248 nm is used as an annealing laser beam for melting the thin film. Also, a laser fluence is made around 500 mJ/cm2.
Besides the above document, there is a method of crystallization wherein, as an annealing laser beam for melting a thin film, a reshaped laser beam in which a xenon chlorine (XeCl) excimer laser having energy of about 1 J per pulse is reshaped to a strip-like elongate beam (350 mm×1 mm=3.5 cm2), and the reshaped laser beam is linearly scanned to irradiate a large-area substrate at a fluence of about 300 mJ/cm2.
A dehydrogenated amorphous silicon thin film having a film thickness of several decades nm is melted by irradiation with an annealing laser beam for several decades to 100 ns, which causes crystallization. The silicon increases its light reflectivity, taking on a metallic nature when melted, and the light reflection intensity of the silicon thin film increases. The method of investigating crystallization as shown in the foregoing document detects by the photo detector a temporal change of the light reflection intensity accompanying the melting of the thin film.
According to the conventional method of investigating crystallization, a location (substantially one position) of an area for melting by annealing laser beam is irradiated by a monitor light, and only the reflected light from the location is detected.
The crystallization of the thin film, namely, the speed and direction of growth of the crystal grain as well as the grain diameter are not uniform actually within an area irradiated by an annealing laser beam. The energy of the annealing laser beam, influenced by the shape of a patterned film, difference in deviation of the film thickness of an amorphous silicon (hereinafter to be called “a-Si”) thin film as a precursor, etc., is not transmitted inside the thin film as previously intended. As a result, the crystal grains are not grown as anticipated, and there used to be caused a dispersion in crystallization of the thin film within the area irradiated by the annealing laser beam.
There is a case where, in order to promote crystal growth of a thin film in the direction of the substrate surface, that is, the lateral direction, a laser energy distribution within the area to be irradiated by the annealing laser beam is intentionally made uneven, or the irradiation pattern of the laser is made asymmetrical. In this case, also, the energy of the annealing laser beam, influenced by the shape of the thin film pattern and others as mentioned above, is not transmitted inside the thin film as previously intended. As a result, the crystal grains were not grown inside the thin film as anticipated.
Due to the dispersion in crystallization of the thin film within the area irradiated by the annealing laser beam, there is caused a difference in investigation results of the crystallization of the thin film depending on which part of the area to be monitored by the monitor light. In other words, the melting area of the thin film is not uniform.
A thin film transistor which has undergone laser-anneal processing on the basis of such an erroneous measurement result of crystallization has a feature out of a predetermined range, and a failure, for example, in electrical feature was caused in a liquid crystal display using this thin film transistor as a switching element or device.
In this way, the conventional art was intended for detecting the crystallization of a thin film by a photo diode on the basis of a piece of information, that is, the information obtained from a reflected light caused by irradiating substantially one-point place of an area for melting the thin film by an annealing energy line (e.g., laser beam).
An object of the present invention lies in accurately observing a crystallization state of a thin film with a monitor light with a high time resolution in real time, which enables to monitor, in an area to be irradiated by an energy line, positions varied from melting to solidification and crystallization.